It might be possible to port clamav; it's open-source AFAIK and runs on Linux. I use it to clean my mail stream mostly to reduce the volume not that I think I'll get infected.

In case anyone gets too cocky about Linux and viruses, there is nothing whatever to prevent a virus being written that exploits .profile, $HOME/bin, cron, etc in userspace, or various application vulnerabilities.
A while ago now you could write an email virus in PostScript - P/S is actually
a scripting language and unless run in safe mode is able to write to the filesystem, e.g. ~/.signature

Right now there is this huge pool of idiots surfing the Web as root in various Microsoft O/S, but that may change.